A Capacitive discharge Ignition System (CDI system) is an electronic ignition system which may be used in spark ignition engines of automotive vehicles. The CDI system uses a capacitor discharge current to fire a spark plug used to ignite a mixture of fuel and air in a combustion chamber of the spark ignition engine.
A typical CDI system consists of a Switch Mode Power Supply (SMPS) that may up convert a DC voltage for example generated in a battery of the automotive vehicle into a high voltage required by an ignition coil connected to the spark plug. The DC voltage may be 12 V while the high voltage supplied to the ignition coil of the spark plug may be typically 150-300 V.
In capacitive discharge ignition systems, the Switch Mode Power Supply (SMPS) is typically a flyback SMPS that uses a transformer and a switch at a primary side of the transformer to perform a so called “flyback” action. The flyback action originates from a reversal of a voltage across a secondary winding of the transformer in consequence to an off state of the switch at the primary side of the transformer. Energy accumulated in the transformer at the primary winding during an on state of the switch, it is released by the transformer at the secondary winding during an off state of the switch. The reversal of the voltage induced across the secondary winding, which is a consequence of a sudden collapse of a magnetic flux in the transformer at the primary winding, is large enough to forward bias a rectifying device arranged between the secondary winding and the capacitor. The rectifying device is used to block currents flowing from the capacitor to the secondary winding during an on state of the switch. By forward biasing the rectifying device, a current may flow from the secondary winding to the capacitor, thereby discharging the transformer at the secondary winding and charging the capacitor. Operation of a flyback SMPS in continuous mode can result in very high currents and heat dissipation. The flyback SMPS is thus typically driven in discontinuous conduction mode to allow a complete discharge of the transformer at the secondary winding during the turn off state of the switch. In the discontinuous current mode the current at the secondary winding falls to zero before the switch is turned on again for another cycle. The discontinuous current mode ensures that no DC current is flowing in the transformer and that all energy stored in the transformer during the turn on state of the switch is transferred to the capacitor during the turn off state of the switch.
In literature many types of flyback switching mode power supplies are disclosed that make use of the discontinuous conduction mode.
For example U.S. Pat. No. 7,719,248B1 discloses a switch-mode converter and a method that uses a sensed current to control the switch-mode converter operating in a discontinuous conduction mode. In one embodiment of the U.S. Pat. No. 7,719,248B1, the switch-mode converter may be a flyback switching mode power supply. The solution provided by U.S. Pat. No. 7,719,248B1 includes a switch-mode converter controller, a comparator and a finite state machine. The comparator receives and compares a sensed current at the primary winding of the transformer with a desired peak current. The finite state machine is configured to operate the switch-mode converter in a discontinuous conduction mode. Responsive to comparisons made by the comparator, the finite state machine turns on the switch and observes an on time duration of the switch until the sensed current reaches the peak current.
Patent application US20080123380 discloses an SMPS and a driving method thereof. The SMPS disclosed in US20080123380 includes a first coil of a primary side of a transformer for transforming an input DC voltage, a second coil and a third coil at a secondary side of the transformer to respectively provide the output voltage of the SMPS and a bias voltage for driving a pulse width modulation signal generator. The pulse width modulation signal generator receives a feedback voltage corresponding to a first voltage generated from the second coil, a sense signal corresponding to the current flowing through the switching transistor, and a third voltage corresponding to a second voltage generated from the third coil. The pulse width modulation signal generator controls an on or off time of the switch so that the SMPS may be driven in a discontinuous conduction mode.
One of the disadvantages of the above mentioned solutions is that a current or a combination of a current and voltages need to be sensed by the SMPS controller in order to drive the SMPS in the discontinuous conduction mode. Since the output of the flyback SMPS is inherently a voltage, extra components are de facto needed in either of the two mentioned solutions. The extra needed components add up to an overall cost of the SMPS. Flyback SMPS used in CDI system are extremely sensitive to cost. A small difference in cost in the order of a few cents may be for example a buying decider of components for increasingly cheaper automotive vehicles containing these CDI systems.
A standard SMPS typically feeds an uncharged capacitive load, which electrically is similar to a short circuit, only at the point of switch on. From thereon the standard SMPS typically delivers a fixed voltage supply to a parallel arrangement of the charged capacitive load and a resistive and/or inductive load. In contrast, in a CDI system the capacitor is discharged up to 200 times a second and thus the standard SMPS spends a significant amount of time operating into a ‘short circuit’. Accordingly, there is a need to control the SMPS in a manner that diverges from traditional means.